Wrap CDUP response path in double-quotes

This allows the pathname to be extracted from the response text.

fix asylumfunk#35
Escape double-quotes in MKD/CDUP/CWD/PWD responses

Per RFC959, double-quotes in response paths should be doubled-up.

fix asylumfunk#36
Remove superfluous parameter

This was not referenced, as CDUP takes no parameter.
Remove superfluous comment

This appears to be the most correct implementation.
Declare and assign variables separately
Bind function scope to caller object

Previously, "self" was being used to maintain a pointer to the "this"
value outside of the anonymous function. Instead, we use the native
Function.bind(...) method to bind the method to the caller's namespace.

Beyond removing a superfluous variable, this also prevents a user from
mistakenly referencing "this" inside the anonymous function instead of
"self".
Rename variables to match convention
Join dangling else clauses
Remove superfluous response

In this case, if we can't find or access a directory, the distinction is
irrelevant.  The generic error message would hint that the directory
actually does exist, but is inaccessible for some other reason
(insufficient permissions, etc.). This behavior could have been
leveraged by a malicious user and should be removed.
Update CWD error log message

Not only is the message shorter, but it also contain the verbatim
command issued by the client.
Return "this" instead of "undefined"

Methods without a return statement (including an empty statement)
implicitly return "undefined", limiting their usefullness. By
returning "this" from any object method without another return value, we
allow method calls to be chained together.
Document CDUP and CWD commands with jsdoc

fix asylumfunk#32
Merge pull request #28 from asylumfunk/i/32

Fix CWD and CDUP